This invention relates generally to surface acoustic wave devices and more specifically to surface acoustic wave devices operated under relatively high power conditions.
As it is known in the art, surface acoustic wave (SAW) devices, such as resonators and delay lines, are used in a variety of applications. Generally, a surface acoustic wave device includes a pair of interdigitated transducers (IDTs) which are the input and output ports of the device with each transducer having a set of conductive members, typically comprised of metal. The conductive members are disposed on, or recessed within, an upper portion of a surface of the device which supports surface acoustic wave propagation.
As it is also known in the art, many applications for radar systems employ SAW devices in oscillator circuits, particularly SAW resonator devices. SAW devices other than SAW resonators, for example SAW delay line devices, could alternately be used in oscillator circuit applications. However, for certain applications SAW resonators are more desirable since they offer a higher quality factor "Q" and therefore better frequency stability characteristics than SAW delay line devices. Such oscillator applications place stringent requirements on the SAW device, particularly with respect to minimizing flicker noise and noise floor levels. Flicker noise refers to the instantaneous fluctuations of the resonant frequency of a device. In a spectral analysis, which represents energy as a function of an offset frequency, "f", flicker noise is the portion of the spectrum which falls off at a rate of 1/f. The oscillator noise floor level is a function of the signal power handling capability of the SAW device. In particular, the noise floor level refers to the ratio of the signal power to the constant thermal, or white, noise level. The larger the signal power, with respect to the constant thermal noise level, the lower the noise floor of the oscillator. In order to meet the stringent noise floor requirements of certain oscillator circuits, the SAW device must operate under relatively high power conditions. For example, the SAW device may be subjected to a typical power level of +18 dBm, this value commonly referred to as the incident power on a device. Approximately half of the incident power on a device (measured in watts) is dissipated, or lost as heat, within the device, and a factor of two decrease in watts is approximately equivalent to a decrease of 3 dB. Thus, given an incident power level of +18 dBm, or approximately 64 milliwatts incident on a device, +15 dBm, or approximately 32 mW, is dissipated within the device.
As it is also known in the art, the high power operating conditions required of SAW devices used in oscillator applications can be detrimental to the long-term frequency stability characteristics of such devices. Long-term frequency stability refers to the variation of the frequency characteristics of a device over time and is typically a critical device parameter. Long-term frequency stability is measured in parts per million per year (ppm/year) and a typical requirement for high precision SAW devices used in oscillator circuit applications is a variation of less than about 1 ppm/year.
Conventionally, in order to provide high power SAW devices having acceptable long-term frequency stability characteristics, the interdigitated transducers of such devices have been comprised of aluminum doped with copper. However, in order to meet more stringent noise floor requirements, particularly of oscillator circuits which utilize SAW devices, such devices must operate with an incident power level of +24 dBm, which is equivalent to +21 dBm (approximately 125 mW) dissipated in the device. While the long-term frequency stability characteristics of conventional SAW devices having copper doped aluminum interdigitated transducers, which are operated at an incident power level of +24 dBm, indicate an overall long-term frequency variation of less than about 1 ppm/year, these devices display significant random fluctuations over shorter durations which is cause for concern.
In articles entitled "A Study of Al-Alloy Electrodes for High Power SAW Filters" by N. Hosaka et al. published in the Japanese Journal of Applied Physics, Vol. 27 (1988) Supplement 27-1, pps. 175-177, "Sputter Deposition for High Power Durable SAW Electrodes" by A. Yuhara et al. published in the Japanese Journal of Applied Physics, Vol. 27 (1988) Supplement 27-1, pps. 172-174, and "Sputtered Al-Ti Electrodes for High Power Durable SAW Devices" by J. Yamada et al. published in the Proceedings of the 1988 Ultrasonics Symposium, Vol. 1 (1988), pps. 285-290, titanium doped aluminum electrodes, or transducers, are used to provide more "durable" SAW devices with respect to stresses and subsequent device failures which generally accompany high power and high frequency operation. Time to failure, or the aging time at which the shift of the center frequency of the device reaches 50 kHz, is one of the parameters used by the above mentioned authors to evaluate the durability of SAW devices. However, while the SAW devices provided by the authors may be suitable for the described portable telephone duplexer application, they do not exhibit acceptable long-term frequency stability characteristics for use in radar system oscillator circuit applications.
It would be desirable to provide SAW devices capable of withstanding high incident power as well as high power dissipation and having improved long-term frequency stability characteristics suitable for use in such applications as radar system oscillator circuits, which demand high power operation as well as stringent long-term frequency stability characteristics of SAW devices.